Conventionally, a motor of a so-called SPM (Surface Permanent Magnet) type is used as a motor for an electric power steering device. The motor of the SPM type includes a rotor and a plurality of magnets. The plurality of magnets is adhered to an outer circumferential surface of the rotor so that two types of magnets having magnetic poles of different orientations are alternately arranged in a radial direction along a circumferential direction. However, in the SPM type motor, when an abnormality occurs in the electric power steering device and the motor becomes inoperable, assist force is not generated by the motor. Instead, brake torque unique to a magnet type motor is generated. As a result, the steering wheel becomes heavy, and steering becomes difficult.
In this regard, taking into consideration that the SPM type motor may become inoperable due to an abnormality in the electric power steering device, a motor of a so-called IPM (Interior Permanent Magnet) type has been proposed in which two types of magnets having magnetic poles of different orientations in the radial direction are alternately arranged in the circumferential direction and embedded in the rotor (for example, Japanese Laid-Open Patent Publication No. 2000-236652). Specifically, in the IPM motor, the embedment of the magnets in the rotor decreases the brake torque that is unique to the magnet type motor generated in an inoperable situation. This results in the steering becoming lighter, and the steering becomes easier.
However, in an IPM type brushless motor capable of reducing the brake torque when the motor is inoperable, the number of magnets is determined in accordance with the number of the magnetic poles. This increases the number of magnets. Accordingly, in the brushless motor used in the electric power steering device, a motor that can replace the motor of the IPM type is desired.